Switchgear and switchboard are general terms which cover metal enclosures, housing switching and interrupting devices, such as, fuses, circuit breakers, relays, along with associated control, instrumentation and metering devices, such as, bus bar, inner connections, and supporting structures, including, assemblies of these devices with associated buses, interconnections and supporting structures used for distribution of electric power.
There are High Voltage switchgear and switchboards, Medium Voltage switchgear and switchboards, and Low Voltage switchgear and switchboards. This invention is primarily geared towards the Low voltage switchgear and switchboards.
Low voltage switchgear and switchboards operate at voltages up to about 635 volts, and with continuous currents that can exceed about 5000 amperes. These Low voltage switchgear and switchboards are designed to withstand short-circuit currents up to about 200,000 amperes.
Low voltage switchgear equipment typically comprises of an assembly composed of multiple metal enclosed sections. Each section may have several circuit breakers stacked one above the other vertically in the front of the section with each breaker being enclosed in its own metal compartment. Each section has a vertical or section bus which supplies current to the breakers within the section via short horizontal branch buses that extend through insulated openings in the rear wall of the breaker compartments. The vertical buses in each section are supplied with current by a horizontal main bus that runs through the line-up. The rear of the section is typically an open area for the routing of cables.
Low voltage switchgear and switchboards are typically designed to withstand the effects of bolted (non-arcing) faults on the load terminals and this capability is validated during Short-Circuit Current and Short-Time Current Withstand Tests in IEEE Standard C37.20.1, the disclosure of which is incorporated herein by reference.
The occurrence of an arcing fault inside the switchgear produces physical phenomena that are different from bolted faults. For example, the energy resulting from an internal arc in air causes a sudden pressure and temperature increase inside the enclosure. Materials involved in or exposed to the arc produce hot decomposition products, both gaseous and particulate, which may be discharged to the outside of the enclosure. This sudden discharge of gaseous and particulate material normally damages the switchgear enclosure and its contents, but may also cause severe injury to an operator who may happen to be nearby.
Arc resistant switchgear qualified to IEEE C37.20.7, the disclosure of which is incorporated herein by reference, is intended to provide an additional degree of protection to the personal performing normal operating duties in proximity to the energized equipment. Accessibility Type 1 arc resistant switchgear has features at the front of the equipment. Accessibility Type 2 arc resistant switchgear has features at the front, sides and rear of the equipment.
Standard metal-enclosed switchgears are designed to withstand the mechanical forces generated by bolted faults on the load terminals until a power circuit breaker or other protective device can interrupt the flow of fault current. This capability is verified by short-circuit and short-time withstand tests on the equipment and interruption tests on the power circuit breakers. During a bolted fault, the voltage at the fault location is essentially zero and the fault energy is dissipated throughout the power system. The arc generated within the power circuit breaker during interruption is cooled and extinguished by the breaker arc chutes. The minimal out gassing of arc byproducts from the arc chutes is contained by the switchgear as verified by interruption tests.
However, it has now been observed that the circuit breaker compartment is also one of the likely places for an arcing fault to occur in switchgear and thus there is a need to address this problem.
An internal arcing fault can be caused by insulation degradation, insulation, contamination, entrance of vermin, foreign objects coming into contact with the energized bus, or any other unplanned condition that creates an electrical discharge path through air. During an arcing fault, the voltage at the fault location is essentially the system voltage and the fault energy is focused within the switchgear enclosure. Arc temperatures can exceed 20,000 degrees Kelvin, rapidly heating the air and vaporizing metal parts. The expanding plasma creates severe mechanical and thermal stress in the equipment which can blow open doors and covers and burn through or fragment the enclosure and/or cause severe injury to an operator who may happen to be nearby.
Thus there is a need in arc resistant switchgear design to provide a means to channel the hot decomposition products created by an internal arcing fault away from the front or the front, sides and rear of the equipment and away from personnel.
There is also a need in many switchgear installations to raise the equipment up off of the floor by the use of a mounting base or housekeeping pad or a sill channel.
This invention overcomes the problems of the prior art and provides a novel method and an apparatus for switchgear assemblies by providing a mounting base with an arc plenum.